ENH: Styler.bar: add support for tablewise application with axis=None #21548

- eliminate code duplication related to style.bar with different align modes
- add support for axis=None
- fix minor bug with align 'zero' and width < 100
- make generated CSS gradients more compact

Author: soxofaan

doc/source/whatsnew/v0.24.0.txt

@@ -256,6 +256,7 @@ Other
 ^^^^^
 
 - :meth: `~pandas.io.formats.style.Styler.background_gradient` now takes a ``text_color_threshold`` parameter to automatically lighten the text color based on the luminance of the background color. This improves readability with dark background colors without the need to limit the background colormap range. (:issue:`21258`)
+- :meth: `~pandas.io.formats.style.Styler.bar` now also supports tablewise application (in addition to rowwise and columnwise) with ``axis=None``
 -
 -
 -

pandas/io/formats/style.py

@@ -984,126 +984,63 @@ def set_properties(self, subset=None, **kwargs):
         return self.applymap(f, subset=subset)
 
     @staticmethod
-    def _bar_left(s, color, width, base):
-        """
-        The minimum value is aligned at the left of the cell
-        Parameters
-        ----------
-        color: 2-tuple/list, of [``color_negative``, ``color_positive``]
-        width: float
-            A number between 0 or 100. The largest value will cover ``width``
-            percent of the cell's width
-        base: str
-            The base css format of the cell, e.g.:
-            ``base = 'width: 10em; height: 80%;'``
-        Returns
-        -------
-        self : Styler
-        """
-        normed = width * (s - s.min()) / (s.max() - s.min())
-        zero_normed = width * (0 - s.min()) / (s.max() - s.min())
-        attrs = (base + 'background: linear-gradient(90deg,{c} {w:.1f}%, '
-                        'transparent 0%)')
-
-        return [base if x == 0 else attrs.format(c=color[0], w=x)
-                if x < zero_normed
-                else attrs.format(c=color[1], w=x) if x >= zero_normed
-                else base for x in normed]
-
-    @staticmethod
-    def _bar_center_zero(s, color, width, base):
-        """
-        Creates a bar chart where the zero is centered in the cell
-        Parameters
-        ----------
-        color: 2-tuple/list, of [``color_negative``, ``color_positive``]
-        width: float
-            A number between 0 or 100. The largest value will cover ``width``
-            percent of the cell's width
-        base: str
-            The base css format of the cell, e.g.:
-            ``base = 'width: 10em; height: 80%;'``
-        Returns
-        -------
-        self : Styler
-        """
-
-        # Either the min or the max should reach the edge
-        # (50%, centered on zero)
-        m = max(abs(s.min()), abs(s.max()))
-
-        normed = s * 50 * width / (100.0 * m)
-
-        attrs_neg = (base + 'background: linear-gradient(90deg, transparent 0%'
-                     ', transparent {w:.1f}%, {c} {w:.1f}%, '
-                     '{c} 50%, transparent 50%)')
-
-        attrs_pos = (base + 'background: linear-gradient(90deg, transparent 0%'
-                     ', transparent 50%, {c} 50%, {c} {w:.1f}%, '
-                     'transparent {w:.1f}%)')
-
-        return [attrs_pos.format(c=color[1], w=(50 + x)) if x >= 0
-                else attrs_neg.format(c=color[0], w=(50 + x))
-                for x in normed]
-
-    @staticmethod
-    def _bar_center_mid(s, color, width, base):
-        """
-        Creates a bar chart where the midpoint is centered in the cell
-        Parameters
-        ----------
-        color: 2-tuple/list, of [``color_negative``, ``color_positive``]
-        width: float
-            A number between 0 or 100. The largest value will cover ``width``
-            percent of the cell's width
-        base: str
-            The base css format of the cell, e.g.:
-            ``base = 'width: 10em; height: 80%;'``
-        Returns
-        -------
-        self : Styler
-        """
+    def _bar(s, align, color='#d65f5f', width=100):
+        """Draw bar chart in dataframe cells"""
+
+        # Get input value range.
+        smin = s.values.min()
+        smax = s.values.max()
+        if align == 'mid':
+            smin = min(0, smin)
+            smax = max(0, smax)
+        elif align == 'zero':
+            # For "zero" mode, we want the range to be symmetrical around zero.
+            smax = max(abs(smin), abs(smax))
+            smin = -smax
+        # Transform to percent-range of linear-gradient
+        normed = width * (s.values - smin) / (smax - smin + 1e-12)
+        zero = -width * smin / (smax - smin + 1e-12)
+
+        def css_bar(start, end, color):
+            """Generate CSS code to draw a bar from start to end."""
+            css = 'width: 10em; height: 80%;'
+            if end > start:
+                css += 'background: linear-gradient(90deg,'
+                if start > 0:
+                    css += ' transparent {s:.1f}%, {c} {s:.1f}%, '.format(
+                        s=start, c=color
+                    )
+                css += '{c} {e:.1f}%, transparent {e:.1f}%)'.format(
+                    e=end, c=color,
+                )
+            return css
+
+        def css(x):
+            if align == 'left':
+                return css_bar(0, x, color[x > zero])
+            else:
+                return css_bar(min(x, zero), max(x, zero), color[x > zero])
 
-        if s.min() >= 0:
-            # In this case, we place the zero at the left, and the max() should
-            # be at width
-            zero = 0.0
-            slope = width / s.max()
-        elif s.max() <= 0:
-            # In this case, we place the zero at the right, and the min()
-            # should be at 100-width
-            zero = 100.0
-            slope = width / -s.min()
+        if s.ndim == 1:
+            return [css(x) for x in normed]
         else:
-            slope = width / (s.max() - s.min())
-            zero = (100.0 + width) / 2.0 - slope * s.max()
-
-        normed = zero + slope * s
-
-        attrs_neg = (base + 'background: linear-gradient(90deg, transparent 0%'
-                     ', transparent {w:.1f}%, {c} {w:.1f}%, '
-                     '{c} {zero:.1f}%, transparent {zero:.1f}%)')
-
-        attrs_pos = (base + 'background: linear-gradient(90deg, transparent 0%'
-                     ', transparent {zero:.1f}%, {c} {zero:.1f}%, '
-                     '{c} {w:.1f}%, transparent {w:.1f}%)')
-
-        return [attrs_pos.format(c=color[1], zero=zero, w=x) if x > zero
-                else attrs_neg.format(c=color[0], zero=zero, w=x)
-                for x in normed]
+            return pd.DataFrame(
+                [[css(x) for x in row] for row in normed],
+                index=s.index, columns=s.columns
+            )
 
     def bar(self, subset=None, axis=0, color='#d65f5f', width=100,
             align='left'):
         """
-        Color the background ``color`` proportional to the values in each
-        column.
+        Draw bars in the cell backgrounds, with a width proportional to
+        the values
         Excludes non-numeric data by default.
 
         Parameters
         ----------
         subset: IndexSlice, default None
             a valid slice for ``data`` to limit the style application to
-        axis: int
+        axis: int or None
         color: str or 2-tuple/list
             If a str is passed, the color is the same for both
             negative and positive numbers. If 2-tuple/list is used, the
@@ -1125,33 +1062,22 @@ def bar(self, subset=None, axis=0, color='#d65f5f', width=100,
         -------
         self : Styler
         """
-        subset = _maybe_numeric_slice(self.data, subset)
-        subset = _non_reducing_slice(subset)
-
-        base = 'width: 10em; height: 80%;'
+        if align not in ('left', 'zero', 'mid'):
+            raise ValueError("`align` must be one of {'left', 'zero',' mid'}")
 
-        if not(is_list_like(color)):
+        if not (is_list_like(color)):
             color = [color, color]
         elif len(color) == 1:
             color = [color[0], color[0]]
         elif len(color) > 2:
-            msg = ("Must pass `color` as string or a list-like"
-                   " of length 2: [`color_negative`, `color_positive`]\n"
-                   "(eg: color=['#d65f5f', '#5fba7d'])")
-            raise ValueError(msg)
+            raise ValueError("`color` must be string or a list-like"
+                             " of length 2: [`color_neg`, `color_pos`]"
+                             " (eg: color=['#d65f5f', '#5fba7d'])")
 
-        if align == 'left':
-            self.apply(self._bar_left, subset=subset, axis=axis, color=color,
-                       width=width, base=base)
-        elif align == 'zero':
-            self.apply(self._bar_center_zero, subset=subset, axis=axis,
-                       color=color, width=width, base=base)
-        elif align == 'mid':
-            self.apply(self._bar_center_mid, subset=subset, axis=axis,
-                       color=color, width=width, base=base)
-        else:
-            msg = ("`align` must be one of {'left', 'zero',' mid'}")
-            raise ValueError(msg)
+        subset = _maybe_numeric_slice(self.data, subset)
+        subset = _non_reducing_slice(subset)
+        self.apply(self._bar, subset=subset, axis=axis,
+                   align=align, color=color, width=width)
 
         return self